Advanced structural ceramic materials have gained the attention of industry by virtue of their superior performance qualities. These qualities, such as superior high temperature strength, high toughness, and resistance to thermal shock and oxidation, provide the bases for their potential use in a variety of applications.
U.S. Pat. No. 4,935,388 (Lucek) suggests that the reliability of a ceramic material can be correlated with its spatial homogeneity, and that such homogeneity can be characterized by the extent to which the material exhibits optical anomalies. In particular, Lucek teaches that the reliability of silicon nitride greatly improves when the material is spatially homogeneous, e.g., has no optical anomalies greater than about 70 microns. Lucek suggests that the above optical anomalies signify a number of inhomogeneous phenomena, including, but not limited to, regions of porosity, regions of contamination from materials such as iron, and regions having microcracks. The silicon nitride ceramics disclosed by Lucek, which contained about 1 w/o MgO as a sintering aid, have a rolling contact fatigue life ("RCF life") defined by an L10 value of at least four million stress cycles in ASTM test STP 771 under 6.9 GPa applied contact stress. Although this RCF life is one of the highest known, commercial desire for silicon nitride materials having improved RCF life continues.
Accordingly, it is the object of the present invention to provide a silicon nitride material having a superior RCF life.